1. Field of the Invention
The present invention generally relates to a rotor of an alternator mounted on a vehicle. More specifically, the invention relates to a magnetic-field rotor having Lundell-type pole cores.
2. Description of the Related Art
As a known technique for improving the output of an alternator mounted on a vehicle, there has been disclosed an "alternator for motor vehicle" in Japanese Patent Laid-Open No. 61-85045. That is, there is such an alternator for a motor vehicle which has a magnetic-field rotor using Lundell-type pole cores, in which a permanent magnet is disposed between pole core claws to increase the effective magnetic flux which contributes to the generation of electric energy, thereby improving output power.
The rotor of the above-described alternator for a motor vehicle, however, has a problem in that since air passage between the claws of the pole core is interrupted, permanent magnets and field coils will become heated with heat generated mainly at the field coils themselves. If the temperature of the permanent magnet rises, the magnetic force decreases, resulting in a lowered effect of prevention of magnetic flux leakage between the claws of the pole cores. Furthermore, with the rise of the magnetic coil temperature, line resistance increases and accordingly, the electric current that flows will decrease, reducing the magnetic flux and consequently the output power.